The present invention relates to a demodulation system for a PSK modulation signal through coherent detection.
Conventionally, a PSK demodulation system must regenerate the reference carrier signal for the purpose of coherent detection.
A reference carrier signal regeneration is carried out by using a narrow band filter or a PLL (phase lock loop) for a multiplied signal of a modulation signal, or by using a costas PLL. When a PLL or a costas PLL is used, a filter is used in a loop for removing phase variation by noise.
FIG. 10 shows a prior art system which uses a frequency multiplier and a narrow band filter. A received signal which is an M-phase PSK signal (M is an integer larger than 2) is applied to the bandpass filter 1. One output of the bandpass filter 1 is applied to the frequency multiplier 50, which provides non-modulated signal having the center frequency M times as high as that of the input signal. The output of the multiplier 50 is applied to the frequency divider 52 which provides the reference carrier signal by dividing the frequency to 1/M through the narrow band filter 51 which reduces noise. A coherent detection is carried out for an input signal by using the .pi./2 phase shifter 60, the multiplicators 61 and 62, and the low pass filters 63 and 64 by using the regenerated carrier signal. The data detect circuit 21 carries out the data detection to provide a demodulated output signal.
FIG. 11 shows another prior art system which uses a multiplier and a PLL. The feature of FIG. 11 is the use of the PLL having the multiplicator 53, the loop filter 54 and the VCO (voltage controlled oscillator) 55, instead of the narrow band filter 51 in FIG. 10. In FIG. 11, the PLL generates the synchronized signal with the multiplied non-modulation signal. The frequency divider 52 provides the divided frequency as the regenerated reference carrier signal.
FIG. 12 is also a prior art system, and is the modification of FIG. 11. The feature of FIG. 12 is that the frequency multiplier 50 and the bandpass filter 1 are included in the loop of the PLL.
FIG. 13 is still another prior art system which uses a costas PLL.
We have proposed the U.S. Pat. No. 4,912,422 as one of the carrier regeneration systems. In that system, an input is quasi-coherent detected, and then, converted to frequency domain signal from time domain signal through FFT (fast Fourier transform). In that system, the frequency and the phase of the multiplied non-modulation signal are estimated on the frequency domain, and the received signal is adjusted by using the estimated frequency and phase, instead of the using a PLL. The coherent detection is carried out by using the adjusted frequency and phase.
By the way, in a satellite communication system, the carrier frequency of a received signal is, generally, different from that of the transmit frequency because of the incomplete frequency stability of each of the stations, and/or the frequency conversion in a satellite. In the case of a low speed digital communication system, which has a narrow bandwidth, the offset of the received frequency due to the above reasons is even four or five times as large as that of the bandwidth of the signal. When the signal has such a large frequency offset, the bandpass filter 1 in FIG. 10 or FIG. 11 must have wide bandwidth for accepting the maximum frequency offset, and therefore, when the C/N (carrier to noise) ratio is low, it is impossible to regenerate carrier frequency.
In the case of FIG. 12 or FIG. 13, it is possible to follow a large frequency offset. However, the range of frequency lock is less than the bandwidth of a loop filter. So, if we try the direct frequency lock, the loop bandwidth must be large, and the C/N of the regenerated carrier signal would be degraded. In order to solve the above problem, it has been proposed to sweep the oscillation frequency by a VCO, however, as the lock-in speed depends upon the sweep speed, and the sweep speed depends upon the bandwidth of a loop filter, the high speed frequency lock is impossible when the C/N is low.
The U.S. Pat. No. 4,912,422 intends to handle only TDMA burst signal, and is not suitable to follow the change of frequency and phase of a continuous PSK signal. Further, as the frequency and the phase of a carrier signal are obtained through interpolation to discrete samples, the accuracy of the result is not enough.